Firefox with the Greasemonkey extension: The Google Calendar TODO list script adds a task list to your calendar with checkboxes to mark items as done.

This todo list isn't as elegant or full-featured as say, Ta-da lists. It's actually quite a hack - but one that works pretty well for a short, ad hoc list. Each task becomes a fake event on your Google Calendar, so you can access your list from any computer that has Greasemonkey with the script installed. How do you incorporate to-do's into your GCal? Let us know in the comments. The Google Cal TODO list script is a free download, and works wherever Firefox does with Greasemonkey.